Print consistency: mains voltage

I did some prints last night and then again tonight; identical exposures (according to my electronic timer) and brand new developer in each case.

The densities are different. Our mains voltage varies a bit and I have a traditional lightbulb enlarger, so my suspicion is that that has caused the exposure error of about 0.15 stops - quite visible in the final print.

So I can see that having some sort of brightness-feedback system as is required for (highly variable) cold heads might also be of some value when printing with tungsten.